Large dynamoelectric machines, such as electric generators, typically utilize a bifurcated cooling system. The interior of the machine is generally cooled by circulation of a pressurized gas coolant within the airtight housing of the machine. Typically, the gas coolant is hydrogen. Other parts of the machine, such as the stator coils, are cooled by the circulation of a liquid coolant, such as water, therethrough. Generally the gas coolant within the frame of the machine is maintained at a higher pressure than that of the liquid coolant used for cooling the stator windings. Typically, the maximum operating pressure of the stator coil water system in the stator coils is about 30 psig, while the turbine generator frame is pressurized to 75 psig with the hydrogen blanket. Therefore, if a leak develops in the stator coil water system within the generator frame, hydrogen gas will leak into the stator coil water system.
Such a situation is to be avoided due to the fact that this in-leakage can tend to decrease the capability of the stator coil water system to function. The largest impact on a stator coil hot spot temperature may result in the case where gas leaks into a stator coil strand at or close to the inlet end (cold) of the coil. Should the leakage rate become sufficiently large, the hydrogen flow in the strand can be in both the upstream and downstream directions, effectively blocking cooling water flow in that strand. Should hydrogen flow back towards the inlet header and enter an adjacent coil strand, the electrical current carrying capacity of the coil may be further reduced. It is therefore important that the hydrogen leakage rate be monitored effectively so that any leaks can be fixed during a normal maintenance outage before significant problems develop.
Due to the pressure gradient of the hydrogen gas being at a higher pressure than the liquid coolant, any leakage is typically detected by the increase in the presence of gas entrained in the liquid coolant system, such as by gas monitor and warning system. The liquid coolant usually flows by means of Teflon.RTM. water hoses located within the generator. Because of this a steady gas leakage rate of up to five cubic feet per day (5 cfd) can be expected in the stator coil water system. Thus, such a monitoring system should compensate for this normal presence of hydrogen gas within the coolant water system. When hydrogen gas leaks into the stator coil water system, it will accumulate in a water storage tank which is part of the stator coil water system assembly. By monitoring the pressure within this holding tank, the leakage rate of hydrogen gas into the stator coil water system can be determined.
Generally, conventional leakage monitors are designed to notify an operator should a gross failure of the water coolant system occur. Thus, this type of system is capable of passing large amounts of hydrogen (on the order of 400 cubic feet per day) before the operator is alarmed to the condition. Small leakage rates per day, which may eventually lead to a gross failure of the system later, can only be detected through close monitoring of a gas meter installed as part of the venting system for the holding tank. Although it is recommended that this gas meter be read and recorded weekly, the meter may be located in a remote spot not readily susceptible to periodic monitoring. Thus, small problems may not be detected before a gross failure can occur. Further difficulty arises in that, although the leak may be at a small but steady rate, it may accumulate in the water tank until it is discharged by a relief valve in large bursts. This can make the task of determining the leakage rate even more difficult. It is therefore desirable to design a system which will more timely alert the operator with an alarm when the hydrogen gas leakage rate exceeds a smaller rate. Such a system should also be capable of being easily retro-fitted, and assist the operator in determining the steady state leakage rate. Such a continuous monitoring system would allow relatively small problems to be monitored more closely so that they can be discovered early on and remedied at a scheduled maintenance outage before any gross failures of the system can occur.